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    <title>Document</title>
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<script>
  function fn(arr) {
    var total = [];
    for (i = 0; i < arr.length; i++) {
      arr[i].length = i + 1;
      var total = total.concat(arr[i]);
    }
    return total;
  }
  var arr = [
    [1, 2, 3, 4, 5],
    [6, 7, 8, 9, 10],
    [11, 12, 13, 14, 15],
    [16, 17, 18, 19, 20],
    [21, 22, 23, 24, 25],
  ];

  console.log(fn(arr));
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